Many attempts have been made to produce orthopedic implants for humans that would be accepted by human host tissue for prolonged periods of time. Some implants often loosen due to the invasion of fibrous tissue while others loosen from the stress of mechanical forces and, eventually, they must be replaced. Other implants--because of the methods used to manufacture them--carry foreign debris when they are implanted which can promote inflammation and ultimately require their removal and replacement.
The response of tissue to any synthetic implant; i.e., any device that comes into contact with soft connective tissue or mineralized tissue, is related to the material used to make the implant, the surface chemistry of the implant and the surface microgeometry of the implant. The effects of tissue response to small changes in the surface chemistry and to small changes in the material used to make the implant are minimal, but such changes in surface microgeometry exert a strong influence on cell behavior.
For example, it has been established that, implants, having relatively high surface free energies as defined by contact angle studies exhibit better in vitro cell attachment and growth characteristics. It has also been established that implants having different surface free energies exhibit similar cell responses in the presence of serum proteins indicating that initial surface free energies are not a dominant factor in tissue response to implants in vivo. [J. Ricci, et al, Bull. Hosp. Joint Dis. Orthop. Inst., 50(2), 126-138 (1990); J. L. Ricci, et al, J. Biomed. Mater. Res., 18, 1073-1087 (1984); J. L. Ricci, et al, J. Biomed. Mates Res., 25(5), 651-656 (1991); J. M. Shakenraad, et al, J. Biomed. Mater. Res., 20, 773-784 (1986); P. vander Valk, et al, J. Biomed. Mater. Res., 17, 807-817 (1983); T. A. Horbett, et al, J. Biomed. Mater. Res., 22, 751-762 (1988); P. B. van Wachem, et al, J. Biomed. Mater. Res., 21, 701-718 (1987)].
Surface microgeometry interaction between tissue and implant surfaces has been demonstrated on ceramic and metallic orthopedic implants. This interaction indicates that smooth implant surfaces promote the formation of thick fibrosis tissue encapsulation and that rough implant surfaces promote the formation of thinner, soft tissue encapsulation and more intimate bone integration. Smooth and porous titanium implant surfaces have been shown to have different effects on the orientation of fibrous tissue cells in vitro. In addition, surface roughness was demonstrated to be a factor in tissue integration into implants having hydroxyapatite surfaces and to alter the dynamics of cell attachment and growth on polymer implants whose surfaces had been roughened by hydrolytic etching. [J. M. Spivak, et al, J. Biomed. Mater. Res, 24, 1121-1149 (1990); J. L. Ricci, et al, "Modulation of Bone Ingrowth by Surface Chemistry and Roughness", The Bone-Biomaterial Interface, University of Toronto press, Toronto, Ont., Can., 334-349 (1991); T. Albrektsson, et al, Acta. Orthop. Scand., 52, 155-170 (1981); T. Albrektsson, et al, Biomaterials, 6, 97-101 (1985); T. Albrektsson, et al, Biomaterials, 7, 201-205 (1986); K. A. Thomas, et al, J. Biomed. Mater. Res., 19, 875-901 (1985); K. A. Thomas, et al, J. Biomed. Mater. Res., 21, 1395-1414 (1987); B. Cheroudi, et al, J. Biomed. Mater. Res., 24, 1067-1085 (1990); T. Inoue, et al, J. Biomed. Mater. Res., 21, 107-126 (1987); U. M. Gross, et al, Trans. Soc., Biomater., 13, 83 (1990); B. R. McAuslan, et al, J. Biomed. Mater. Res., 21, 921-935 (1987)].
From the examination of in vitro growth characteristics of normal cells cultured on flat surfaces there has evolved the following cell "behavioral" characteristics:
attachment dependent growth; i.e., the dependence of normal diploid cells or substrate attachment for normal growth; PA1 density-dependent inhibition; i.e., the tendency of such cells to slow or stop growing once a confluent monolayer is formed; PA1 substrate-exploring function; i.e., the ability of some types of cells to migrate on a surface in search of acceptable areas for attachment and growth; and, PA1 contact guidance; i.e., the ability of some types of cells to migrate and orient along physical structures. PA1 (a) promote the rate and orient the direction of bone growth and discourage the growth of soft tissue to achieve secure fixation of the implant surfaces to bone tissue; PA1 (b) promote the rate and orient the direction of the growth of soft tissue while discouraging the growth of bone tissue to achieve soft tissue integration with the implant surfaces; and, PA1 (c) create a barrier that discourages the growth of soft tissue, particularly soft fibrous tissue, and thereby prevent the migration of soft tissue growth into the bone tissue attachment surfaces of the implant.
[J. L. Ricci, et al, Trans. Soc. Biomat., 17, 253 (1991); J. L. Ricci, et al, Tissue-Inducing Biomaterials, Mat. Res. Soc. Symp. Proc, 252, 221-229 (1992); J. Ricci, et al., Bull. Hosp Joint. Dis. Orthop Inst., supra; J. L. Ricci, et al, J. Biomed Mater Res., 25(5), supra; M. Abercrombie, et al, Exp. Cell. Res., 6, 293-306 (1954); M. Abercrombie, Proc. Roy Soc., 207B., 129-147 (1980); D. M. Brunette, et al, J. Dent. Res., 62, 1045-1048 (1983); D. M. Brunette, Exp. Cell. Res., 164, 11-26 (1986); P. Clark, et al, Development, 108, 635-644 (1990)].
The behavioral characteristic of cellular contact guidance has been demonstrated in vitro on a variety of surfaces such as grooved titanium, grooved epoxy polymer, and collagen matrix materials of different textures and orientations. Grooved machined metal and polymer surfaces have also been shown to cause cellular and extracellular matrix orientation in vivo and be used to encourage or impede epithelial down growth in experimental dental implants. [B. Cheroudi, et al, J. Biomed. Mater. Res., 24, 1067-1085 (1990) and 22, 459-473 (1988); G. A. Dunn, et al, supra; J. Overton, supra; S. L. Shor, supra; R. Sarber, et al, supra].
Substrates containing grooves of different configurations and sizes have been shown to have orientating effects on fibroblasts and substrates containing grooves of varying depths have been shown to have different degrees of effect on individual cell orientation establishing that grooved surfaces can modulate cell orientation in vitro and can cause oriented cell and tissue growth in vivo. For example, it has been shown that fibrous tissue forms strong interdigitations with relatively large grooves in the range of about 140 .mu.m and can result in an effective barrier against soft tissue downgrowth perpendicular to the grooves. It has also been shown that smaller grooves on the order of about 3-22 .mu.m were more effective in the contact guidance of individual cells. [D. M. Brunette, et al. Development, supra; P. Clark, et al, supra.]
These data have given rise to the development of several different types of implants. For example, U.S. Pat. No. 4,608,052 to Van Kampen discloses implants made from different compositions. The implants have attachment surfaces comprising a plurality of spaced posts and interconnecting, partially spherical, concave surfaces. The exemplified implants are stemmed femoral components which were produced using lasers to create posts having different geometric configurations. The implants are stated to be an improvement over smooth surfaced implants as the posts and concave surfaces permit interlocking tissue growth and intimate approximation with the implant surfaces to provide a relatively high degree of frictional "fit" between the implant and the human host tissue.
U.S. Pat. No. 5,002,572 to Picha discloses implants having texturized surfaces which are stated to be useful as mass transfer devices and which are intended to be implanted in soft tissue. The texturized surfaces of these implants comprise micro pillars having specified widths and spacing and minimum heights. The implants can be provided with means to deliver a therapeutic agent to the implant site to ward off infection and enhance tissue growth.
Implants also intended for use in soft tissue are disclosed in U.S. Pat. No. 5,011,494 to von Recum, et al. The texturized surfaces of these implants have a variety of geometric configurations comprising a plurality of projections and recesses formed in a three dimensional implant body. It is specified that the mean bridging, breadth and diametric distances and dimensions as defined are critical; i.e., a minimum defined depth must be present and the limiting dimension factor is the smallest, not the largest, dimension. It is also stated that the disclosed implants promote anchorage and the growth of collagen at the implant site "without causing encapsulation of the embedded portion of the device".
In general, the foregoing publications establish that cell attachment, growth, migration and orientation, as well as extracellular matrix synthesis and orientation, are moderated by substrate surface shape (i.e., microgeometry) as well as by surface chemistry. However, the findings in these publications do not address what effects different substrate microgeometrics and sizes would have on various cell colony growth and migration parameters as opposed to the morphology of individual cells. Thus, while these publications establish that surface microgeometry of implants influences cell orientation, they do not disclose or suggest what effect different surface microgeometry,of implants would have on both the rate and direction of the cell colony growth of different cells and different tissues surrounding an implant.
Current methods used to texturize the surfaces of implants typically employ sand, glass bead or alumina grit blasting techniques and acid etching techniques of the implant surface. In sand, glass bead or alumina grit blasting techniques, compressed air is generally used to drive a blasting medium onto the implant surface at a high velocity in order to deform and, in some instances, remove portions of the implant surface. The surface texture obtained depends upon the size, shape and hardness of the implant material and on the velocity at which the blasting medium is driven onto the implant surface. The most common surfaces produced by sand or glass bead blasting are matte or satin-finishes while alumina grit blasting produces a roughened surface.
In acid etching techniques a pattern or mask is placed upon that surface of the implant desired to be texturized. The pattern or mask is then typically treated with an acid that corrodes the exposed surface of the implant whereupon the pattern or mask is removed and the acid treated surface is washed or neutralized with an appropriate solvent.
Illustrative of the sand or glass bead blasting technique is the method disclosed in U.S. Pat. No. 5,057,108 to H. R. Shetty, et al wherein the implant surface is shot blasted with metal shot followed by glass bead blasting and then electropolishing.
Illustrative of an acid etching technique is the method disclosed in U.S. Pat. No. 4,778,469 to R. Y. Lin, et al wherein an acid soluble (e.g., aluminum or zinc) space occupier is used. The space occupier contains the pattern to be transferred to the implant surface and is placed on the desired portion of the implant surface that is to be texturized. The space occupier is pressed into the implant surface and is then removed by treating it with acid. The materials used as implants are thermoplastic resins such as polyetheretherketone and polyphenylene sulfid.
It has been found that these typical blasting techniques leave debris from the processing materials embedded in the implant surface as contaminants. This debris has also been found in soft tissue isolated from the areas adjacent to failed press-fit total hip replacements indicating that the debris was released from the surface of the implant.
These problems of residual contaminant debris are overcome by using the system and assemblage of the invention which produces texturized implant surfaces without introducing embedded, particulate contaminants. In addition, the system and assemblage of the invention can be used to remove contaminants from implant surfaces that have been texturized by blasting employing optical and/or chemical lithography and acid etching techniques such as by ablating the treated surface. During ablation, the treated surface can also be annealed to enhance the fatigue properties of implants produced from metals.